The present invention relates to the packaging of integrated circuits (ICs) and more particularly to a method of forming a pre-molded lead frame used to form a semiconductor package.
A lead frame is a structure that provides electrical interconnections to a semiconductor die. That is, a die is attached to a lead frame and then bonding pads of the die are electrically connected to leads of the lead frame, typically with wires via a wire bonding process. The die, lead frame and wires are then encapsulated with a plastic mold compound. Exposed areas of the leads provide the electrical interconnection to the die.
In order to provide good electrical connection to other devices, certain portions of the leads must be adequately exposed. If this lead area is covered with mold compound or otherwise not well exposed or easily contactable then issues can arise such as solderability of the leads to a substrate or printed circuit board (PCB).
FIGS. 1-4 illustrate steps in a semiconductor packaging process. FIG. 1 shows a pair of lead frames 10 including half-etched portions 12. Integrated circuits 14 are attached to flags 16 of the lead frames 10. The integrated circuits 14 are electrically connected to leads 20 of the lead frames 10 with wires 18, typically via a wire bonding process. In the drawing, the two lead frames 10 are part of a strip of lead frames. The strip of lead frames is secured to a tape 22. FIG. 2 shows the lead frames 10 located inside a mold chase 24. The arrows indicate a direction of mold flow. FIG. 3 shows a mold compound 26 that has been injected into the mold chase 24. The mold compound 26 covers the lead frames 10, the integrated circuits 14 and the wires 18.
FIG. 4 is an enlarged view of a contact area of one of the lead frames 10, particularly, the half-etched portions of the flag 16 and lead 20. Note that the mold compound 26 fills the space between the flag 16 and the lead 20, and further, the mold compound 26 and the bottom surface of the lead frame are planar. It would be advantageous to be able to increase the stand-off between the lead frame and the mold compound.